Well, if she's never seen you in it, and all she's heard from others is that you're wearing a skirt, then maybe it's time for her to see how you look in said "skirt". Once she sees for herself that you aren't trying to look like a school-girl, she's likely to change her opinion -- or not. But at least then her opinion is based on something other than hearsay and you can have a realistic conversation about it.